1. Field of the Invention
This invention relates to a system for detecting closed contacts in a matrix switch.
2. Description of the Prior Art
A matrix switch formed by arranging a plurality of switch contacts in the shape of an XY matrix by using a transparent conductive sheet is generally known.
FIG. 1 is a sectional view showing the construction of such a matrix switch. Referring to FIG. 1, reference numerals 1, 2 denote transparent conductive sheets in each of which a plurality of conductive layers 3 are formed in parallel with one another on a transparent sheet member of a synthetic resin. Since the conductive layers 3 are formed to a small thickness by evaporation, even the portions of the transparent conductive sheets 1, 2 which correspond thereto are transparent. These transparent conductive sheets 1, 2 are laminated on each other with the conductive layers 3 positioned on the inner side of the resultant lamination in such a manner that the conductive layers 3 on one transparent sheet member cross those on the other; the lamination as a whole constitutes a transparent structure. The conductive layers 3 in one transparent conductive sheet are used as sense lines (X lines), and the conductive layers 3 in the other as driving lines (Y lines). The intersections of these conductive layers function as contacts in a switch. Such a matrix switch is placed on a display panel on a light-emitting display, such as a plasma display, and used as an input device of the type in which information is inputted by pressing by a finger the intersections corresponding to letters and symbols indicated on the display panel.
Such a matrix switch generally employs a sense system, in which the sense lines (X lines) are pulled up to a predetermined electric potential through resistors to monitor variations in the electric potentials of the sense lines while grounding the driving lines (Y lines) in order by a switching element or connecting the driving lines to predetermined electric potential points (scanning), to thereby detect closed contacts.
However, when this sense system is applied to a switch matrix having a high resistance value (for example, 100 K.OMEGA.) while the switch is on, such as a switch matrix using indium oxide as an evaporation material for its conductive layers, the scanning speed is restricted greatly or closed contacts are detected erroneously. Namely, when a resistance value is high while the switch is on, it is necessary to increase the resistance values of the pulling-up resistors as well for increasing the amplitude of the electric potential of the sense lines. This causes the time constant of a circuit including the sense lines to increase. Consequently, after the scanning of a certain closed contact has been completed, the electric potential of the sense line connected to the closed contact is not recovered immediately, and, therefore, when the scanning time is short, another contact connected to the same sense line is detected erroneously as a closed contact during the scanning thereof.